1. Field of the Invention
The present invention relates to a fluid circulation apparatus used for passing fluid from an interior of a drill string to the wellbore's annulus upon command from the surface and, more particularly, to a fluid circulation apparatus that can be used in directional drilling.
2. Description of Related Art
With the world's known oil reserves declining, extraordinary efforts are being made to continue oil production from existing oil fields. One such method is to drill a wellbore at an incline or even horizontally to reach oil trapped in relatively small pockets. Also, a wellbore can be drilled laterally from an existing wellbore to intersect one or more subterranean faults which can permit additional trapped oil to flow to the wellbore for recovery. The art of being able to steer or guide a drill string at an incline or horizontally to a desired location is usually referred to as "directional drilling". To understand the location of the advancing drill bit from the earth's surface, a directional driller uses various techniques. In some cases, acoustical pulses in the drilling mud are measured and in others sensitive electronic downhole telemetry (telemetric) devices are utilized.
A circulation valve is used to redirect the flow path of drilling fluid, to enable the drill bit to clear debris, drill cuttings, sloughed formation particles or other such unconsolidated particles which may be restricting movement of either the drill string, or drilling mud from the bit. Since directional drilling is dependent upon downhole motors operated by flowing mud, the circulation valve is necessary to maintain circulation in the drilled interval while the drilling motor is stopped. For this reason it is necessary to be able to close the circulation valve, and reopen it intermittently while drilling. A circulation sub provides a controllable opening so that drilling fluid can be passed from the inside of the drill string to the wellbore's annulus. Typically, circulation subs are mechanically actuated by the dropping of a metal bar or plug within the drill string that causes a localized fluid pressure increase that opens the circulation ports. This type of prior circulation sub is shown in U.S. Pat. No. 3,941,190. This prior circulation sub has the disadvantage of requiring the operator to retrieve or "fish" out the bar or ball before drilling can continue. This prior circulation sub will not close, so it is non-resettable. Additionally, in horizontal wellbores, the ball or bar most likely will not pass downhole to the circulation sub due to the lack of gravity assistance in the horizontal sections of the wellbore.
Other circulation subs that do not require the use of a dropped ball or bar utilize internal pressure relief valves, as shown in U.S. Pat. Nos. 2,833,517 and 4,768,598, acoustic signals, as shown in U.S. Pat. No. 4,373,582, and a dedicated hydraulic control line, as shown in U.S. Pat. No. 5,236,047 (which is commonly assigned hereto). The circulation sub shown in U.S. Pat. No. 5,236,047 utilizes the application of hydraulic fluid through a dedicated control line to open the circulation ports in the circulation sub to permit the fluid to escape to the annulus.
Directional drilling systems will often utilize extremely sensitive downhole electronic measuring devices (often called Measurement-While-Drilling equipment or "MWD") to enable the operator at the earth's surface to determine the location of the advancing drill string and its direction of advancement. Due to the extreme sensitivity of the MWD equipment, other downhole equipment must be designed to not interfere with the MWD equipment. While the circulation sub shown in U.S. Pat. No. 5,236,047 can be used in highly deviated wellbores and adjacent the extremely sensitive MWD equipment, it does require the use of a dedicated source of hydraulic fluid to operate, which may not be feasible if other hydraulically operated downhole tools are to be operated from the same hydraulic fluid source.
There is a need for a circulation sub that can be used in highly deviated wellbores and adjacent MWD equipment, and which can be actuated from the earth's surface by a signal separate from fluid which is used to open or close the circulation ports.